1. Field of the Invention
The present invention relates to an immunometric assay using monoclonal antibodies for the detection or determination of human chorionic gonadotropin.
2. Brief Description of the Background Art
Human chorionic gonadotropin (hCG) is a placental glycoprotein hormone composed of two non-identical alpha and beta subunits. Tyrey, L., Semin. Oncol. 9:163 (1982). The alpha subunit of hCG is virtually identical to the alpha subunits of thyroid-stimulating hormone (TSH), luteinizing hormone, (LH) and follicle-stimulating hormone (FSH). The unique biologic and immunologic reactivity of these hormones, however, is defined by their beta subunits, which are structurally different for each hormone. The 145 amino acid beta subunit (beta hCG) of hCG has over 80% homology with the beta subunit (beta LH) of LH. The single major structural difference between the beta subunits of these two hormones is the presence in beta hCG of an extra COOH-terminal extension (residues 115-145), which is absent from beta LH. Morgan, F. J. et al., J. Biol. Chem. 250: 5247 (1975). hCG and LH display similar immunoreactivity due, in part, to their extensive amino acid sequence homology. Bidart, J. et al., J. Immunol. 134 (1):457 (1985).
Because of these extensive chemical homologies between hCG and LH, specific detection of hCG is problematic. For example, both antisera and monoclonal antibodies (Mabs) elicited against hCG cross-react with LH. Tyrey, L., supra. Moreover, radioimmunoassays (RIAs) presently in clinical use cannot easily detect low levels of hCG (less than 0.5 ng/ml) without concentration of the biological sample. Borkowski, A. et al., J. Clin. Endocriol. Metab. 58:1171 (1984). These limitations have hampered studies on hCG production in healthy individuals. Borkowski, A. et al. supra; Borkowski, A. et al., N. Engl. J. Med. 301:298 (1979). Also, it is known that certain neoplasms secrete hCG. Thus, hCG can be used as a "marker" for diagnosing or monitoring treatment of some human neoplasms. Tyrey, L., supra. However, more sensitive assays for hCG are required to detect early recurrance following treatment for trophoblastic tumors.
To achieve better specificity and sensitivity in the immunologic recognition of hCG, several attempts have been made to produce antibodies directed against the unique beta-COOH-terminal structure of hCG (residues 115-145) by using as immunogens C-terminus peptides (CTP) of the hCG beta subunit or C-terminus synthetic peptide analogues. See, for example, Louvet, J.P. et al., J. Clin. Endocrinol. Metab. 39:1155 (1974); Matsuura, S. et al., Endocrinol. 104:396 (1979); Birken, S. et al., Endocrinol. 110:1555 (1982). Antisera and monoclonal antibodies thus produced were, however, of low affinity, and the sensitivity of the assays was poor or offered no advantages compared to antibodies raised against the entire hCG subunit. Birken, S. et al., "Immunochemical Recognition of Human Choriogonadotropin," in, Burchiel, S. W. et al., (Eds.) Tumor Imaging, Masson Publishing, U.S.A., Inc., New York, p. 41 (1982); Bellet, D. et al., Endocrinol. 115:330 (1984); Bidart, J.M. et al., J. Immunol. 134:457 (1985).
Thus, although several hCG immunoassays based on monoclonal anti-CTP antibodies recently have been described, Bidart, J.M. et al., supra; Caraux, J. et al., J. Immunol. 134:835 (1985), the sensitivity of these assays is similar to that obtained with antibodies raised against the intact hormone. Similarly, Canfield et al., PCT Publication No. 84/04598, disclose an hCG immunoassay utilizing a polyclonal antibody to the carboxy terminal peptide of the beta hCG subunit in conjunction with a Mab to the beta hCG subunit, which has the disadvantages associated with the use of polyclonal antibodies. A need, therefore, has continued to exist for a highly sensitive and specific assay for hCG.